FirebaseAuth.getInstance() returns null

I have a problem with getting mAuth.getInstance() from Firebase. It always returns null. Because of this problem, the whole app crashes and I cannot move on. How can I solve this problem?

I've tried to modify gradle versions, but those did not solve anything. I added the necessary code snippets here.

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.smartfarmandroidapp"
    xmlns:tools="http://schemas.android.com/tools">

    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        tools:node="replace"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.SmartFarmAndroidApp">
        <activity
            android:name=".MVVM.View.Activities.DrawerActivity"
            android:label="Drawer Activity"
            android:theme="@style/AppTheme.NoActionBar"
            android:windowSoftInputMode="adjustPan"/>
        <activity android:name=".MVVM.View.Activities.MainActivity" />
        <activity android:name=".MVVM.View.Activities.LoginActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Gradle :module

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.example.smartfarmandroidapp"
        minSdkVersion 16
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"
        multiDexEnabled true
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.3.0'
    implementation 'com.google.android.material:material:1.3.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'

    //Firebase Authentication
    implementation platform('com.google.firebase:firebase-bom:28.0.1')
    implementation 'com.firebaseui:firebase-ui-auth:7.1.1'
    implementation 'com.google.firebase:firebase-auth'
}

Gradle :project

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        google()
        jcenter()
        mavenCentral()
    }


    dependencies {
        classpath 'com.android.tools.build:gradle:4.1.2'
        classpath 'androidx.navigation:navigation-safe-args-gradle-plugin:2.3.5'
        classpath 'com.google.gms:google-services:4.3.8'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

LoginActivity.java

 private static final int SIGN_IN = 42;
    private static final String TAG = "LoginActivity";
    private FirebaseAuth mAuth;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FirebaseApp.initializeApp(this);
        mAuth = FirebaseAuth.getInstance(); //This line returns null 
    }

    @Override
    public void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
    }

The entire error exception which I get is the following:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.smartfarmandroidapp, PID: 16328
    java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.auth.FirebaseUser com.google.firebase.auth.FirebaseAuth.getCurrentUser()' on a null object reference
        at com.example.smartfarmandroidapp.MVVM.View.Activities.LoginActivity.onStart(LoginActivity.java:191)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435)
        at android.app.Activity.performStart(Activity.java:8024)
        at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3475)
        at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
        at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)